1. Field
The present invention generally relates to apparatus and method for saving power in mobile units and more particularly to saving power in stereo audio codecs utilized in mobile units.
2. Related Art
Traditionally, mobile units, such as cellular phones, primarily used analog communications, such as FM, for radio transmission. Currently, digital communications, such as code division multiple access (“CDMA”), are the dominant form of transmission because they allow more users to occupy the bandwidth, and at the same time improve voice quality and reliability.
For digital communication techniques to be used on the reverse link, audio signals, such as voice, must be converted from an analog signal to a digital signal. Conversely, on the forward link, the received digital signal must be converted to an analog signal in order to be played by the mobile unit speaker. An audio coder/decoder capable of performing both directions of analog/digital conversion is commonly referred to as a “codec.” Codecs are in general use in various mobile units, such as cellular phones.
As is known in the art, mobile units, such as cellular phones, are generally battery operated. Extended battery life is a desired feature in today's cellular phone market. In addition to long battery life, an attractive product must have the latest features, such as capability to play music, to be competitive with competing manufacturers.
New features such as music capabilities, in which users can listen to MP3 audio, are becoming increasingly popular in current cellular phones. Such devices require stereo codecs as opposed to mono codecs found in older cellular phones.
The exemplary stereo audio codec 101 in FIG. 1 can be part of a digital cellular phone that can play stereo audio. Stereo audio codes 101 converts audio analog signals to digital signals for transmission in the reverse link. Stereo audio codec 101 is also used for converting digital signals to audio analog signals for outputting the caller's voice on the phone's speaker in the forward link, or for playing audio.
Exemplary stereo audio codec 101 consists of two audio processing paths 116 and 118. The transmit audio processing path 116 is used, for example, to convert the analog voice signal of a user to a digital voice signal. A brief description of transmit audio processing path 116 follows.
Voice signal input can originate from the mobile unit microphone 132, a headset microphone 134, or an auxiliary input 136, which is connected to stereo audio codec 101 by way of an I/O jack 128. I/O jack 128 provides input to a first microphone amp 138 that amplifies the input analog signal. The output analog signal of first microphone amp 138 may optionally be amplified again by a second microphone amp 142 and/or filtered by high pass filter and gain 144 depending on the type of input. For example, optional filtering may be performed on the auxiliary input 136.
Mux 146 may select output of high pass filter and gain 144 or it may select output of first microphone amp 138. The output of mux 146 is coupled to analog-to-digital converter 148, which converts an inputted analog voice signal to a digital voice signal. Signals to the right of divider 105 are digital signals. The digital voice signal is then filtered by transmit filter 150 and amplified by transmit gain 152 to produce transmitted voice pulse code modulation (“PCM”) 153. Transmitted voice PCM is sent to PCM interface 122, which is then sent for further processing such as voice compression through PCM “input” signal 154.
Receive audio processing path 118 is similar to transmit audio processing path 116 except that the signals flow in the opposite direction and that there are two signal paths for the left and right channel. PCM “output” signal 155 interfaces stereo audio codec 101 by way of PCM interface 122. The “output” in PCM “output” signal 155 refers to “output samples” that ultimately arrive at one or more speaker outputs, either earpiece speaker 123, left headset speaker 124, and/or right headset speaker 126.
If the PCM “output” signal 155 is stereo, PCM interface 122 splits the signal between left channel digital signal 156 and right channel digital signal 157 for separate processing. The left channel digital signal 156 is amplified by receive gain 162 and filtered by receive filter 164. The output of receive filter 164 is then converted to an analog output through digital-to-analog converter 170. Signals to the left of divider 105 are analog signals. The same processing of receive gain 166, receive filter 168, and digital-to-analog converter 172 is performed for the right channel as well.
Outputs of left and right channel digital-to-analog converters 174 and 176, respectively, provide inputs to audio output mux 178. Audio output mux 178 directs analog outputs to either earpiece amp 181 or left and right headset amps 180 and 182, respectively, depending on what type of audio output device is connected or not connected to I/O jack 128.
Stereo audio codecs consume substantially more power than mono audio codecs, which were used on previous generations of cellular phones. Stereo audio codecs can consume approximately 45% more power than mono audio codecs because they have an extra audio processing path to produce both left and right channels, as shown in receive audio processing path 118.
If PCM output signal 155 is mono and a mono headset is attached, it is desirable to only have one of the channels in the receive audio processing path 118 enabled (i.e., turned on) and the other channel disabled (i.e., turned off). For example, if the right channel is the “non-default” channel, it is desirable to have the right channel containing components such as receive gain 166, receive filter 168, and digital-to-analog converter 172 disabled.
However, according to known techniques employed in stereo audio codecs, there is no method of conserving power by shutting down audio processing components of the “non-default” channel such as receive gain 166, receive filter 168, and digital-to-analog converter 172 in the right channel.
Also, when stereo headset speakers are connected to I/O jack 128 and PCM output signal 155 is mono, it is abnormal for the user to hear audio on only one speaker of a stereo headset if PCM output signal 155 is mono. For system 101 in FIG. 1, a known method of placing a mono signal on both speakers is to simply duplicate the mono signal on left and right channels at 156 and 157. In this method, left channel receive gain 162, receive filter 164, and digital-to-analog converter 170 will be on as well as corresponding right channel components receive gain 166, receive filter 168, and digital-to-analog converter 172. However, this method, although easy to implement, consumes power for both stereo processing paths.
Unless novel techniques for conserving power in a stereo audio codec are used, mobile units, such as cellular phones, will have shorter battery life, which may lead to poor sales and user dissatisfaction. There is therefore a need in the art for efficient power management for stereo audio codecs in mobile units, such as cellular phones.